Podocyte foot processes (FP) and the interposed slit diaphragms (SD) form the final barrier to protein loss, explaining why podocyte injury is typically associated with marked proteinuria. The highly dynamic FP actin cytoskeleton is linked to the SD and proteins regulating podocyte actin dynamics are therefore of critical importance for structural maintenance and sustained function of the glomerular filter. Synaptopodin (synpo) was first described by the PI as the founding member of a novel class of actin associated proteins'highly expressed in podocytes and telencephalic dendrites. Based on the data obtained with this grant, we established a novel paradigm for the regulation of podocyte FP actin filament formation by synpo, which interacts with and modifies the function of a-actinin-4, the target gene of an autosomal dominant form of human focal segmental glomerulosclerosis (FSGS). To test this hypothesis, we propose the following three Specific Aims: Specific Aim 1 will elucidate the cellular mechanism by which synpo regulates the formation and dynamics of actin filaments in podocytes. Specific Aim 2 will explore at the molecular level how synaptopodin cooperates with alpha-actinin-4 to elongate a-actinin-4 induced actin filaments. In Specific Aim 3 we will explore the structural and functional consequences of the combined deletion of Synpo-long and Synpo-T in mice. If our hypothesis is correct, the work proposed here will have broad significance because it will provide us with a better understanding of the biological mechanism underlying the development and dynamic re-organization of the mature podocyte FP actin cytoskeleton under physiological conditions and in nephrotic syndrome. This should in the long-term enable us to develop novel, selective podocyte-protective therapies that tackle proteinuria by promoting the synpo/ a-actinin-4 mediated recovery and maintenance of the normal FP architecture.